The present invention relates in general to local area networks and, more particularly, to method and apparatus for maintaining a dynamic logical ring in a token passing local area network.
Local area networks (LANs) are becoming more and more prevalent in today's world. With this widespread acceptance comes the need to improve both the speed and efficiency of these systems. The present invention provides method and apparatus for improving the speed and efficiency of a token passing LAN by maintaining a dynamic logical ring.
In a token passing local area network, a plurality of bus interface units (BIUs) are coupled to a common transmission line for transmitting information packets therebetween. Each BIU, or node, is permitted to transmit only while in control of the token. The token is passed from BIU to BIU in accordance with the order of the logical ring.
Logical ring, as used herein, refers to a list which prescribes the order in which the token is passed from BIU to BIU. In most LANS the logical ring need prescribe no particular order in which to pass the token, the only limitation being that each BIU listed in the logical ring must be able to receive the token and, thereafter, transmit the token.
In token passing local area networks, it is desirable to maintain a dynamic logical ring, i.e., a logical ring which only passes the token to active BIUs, thereby reducing the token rotation time. However, prior art protocols, such as that described in IEEE Project 802 Local Network Standards, Draft C, Section 4 (1982), for adding or removing BIUs to and from the logical ring incur delays caused by the departing, entering and soliciting BIUs which make the mean bus access delay unacceptable for real time operations.
Particularly, prior art protocols require that the entering BIU be solicited by a participating BIU. Thus, a BIU wishing to enter the ring cannot unilaterally initiate participation in the local area network. Further, a BIU wishing to enter the network can only be solicited by a particular one of the BIUs already participating therein. Hence, additional delay is incurred until that particular BIU solicits prospective entrants to the ring.
Also, prior art protocols for adding and removing BIUs to and from the logical ring incur additional delays in information transfer when more than one prospective entrant responds to an entrant solicition, i.e., a collision occurs. In such instances, information transfer is suspended while the several responding prospective entrants determine which will be added to the logical ring. Hence, prior art protocols incur substantial delays in information transfer during the intervals when a contention for entrance between several BIUs is being resolved.
Similarly, when a BIU fails, and therefore cannot transmit the token to its successor, the logical ring must be re-established. Prior art protocols for reestablishing the logical ring incur unacceptable delays in determining the new successor to the failed BIU's predecessor, i.e., re-establishing the ring. This is especially so when two or more successive BIUs fail simultaneously.